One of the salient features of the vertebrate central nervous system (CNS) is the incredible complexity of cell types. The developmental processes that generate such diversity are as yet quite unexplored. A model system in which these processes can be studied in vitro and in vivo would greatly aid efforts aimed at understanding the molecular mechanisms that govern determination of cell fate. The retina offers such an opportunity. It is relatively simple, well-characterized, and classically has served as a model for CNS development and function. We have devised an in vitro system that allows the generation of postnatal retinal cell types, including rod photoreceptors, bipolar cells, muller glial cells, and amacrine cells. Mitosis of uncommitted progenitors, commitment to cell fate, and at least partial differentiation occur within dissociated primary cells explanted from rat retina and cultured in serum-free, defined medium. We propose to study these processes in vitro, focussing on the role of intrinsic and extrinsic cues. The developmental regulation of progenitor responsiveness, and of production of factors that are required for these events, will be studied. As the assay system is robust (in vivo rates of generation of rods, bipolars, and muller glia have been achieved), we also plan to isolate and identify proteins and/or genes encoding these activities. Our initial efforts will be on the rod photoreceptor pathway. Information concerning de novo rod generation may be relevant to rod survival, rod regeneration, or rod replacement and thus useful in the understanding and/or treatment of degenerative diseases such as retinitus pigmentosa.